1. Field of the Invention
This invention relates to an image forming apparatus such as a copying machine, a facsimile apparatus or a printer utilizing the electrophotographic system, the electrostatic recording system, the heat transfer system or the like, to form images on recording materials and thereby obtain hard copies.
2. Related Background Art
An image forming apparatus such as an electrophotographic apparatus usually has, as the conveyance path of a recording material, a series of conveyance paths for feeding, transferring, fixing and discharging of the recording material having an image formed thereon out of the apparatus. In this case, any difference between the conveyance speeds of elements constituting the conveyance path has sometimes reduced the quality of the image formed. For example, in an electrophotographic apparatus as shown in FIG. 5 of the accompanying drawings, a toner image formed on a photosensitive drum 102 as an image bearing member is transferred to a recording material fed from a cassette 106, at a transfer station 104, by the action of a charger 103. By the corona discharge produced from the charger 103 during the transfer, coupled with the movement of the photosensitive drum 102, the conveyance force by the photosensitive drum for the recording material is created. This conveyance force results from electrostatic attraction and is very small as compared with the conveyance force provided in an apparatus of a construction in which a plurality of rollers such as a pair of conveying rollers 110 and a pair of fixing rollers 101 are urged against each other and a recording material is conveyed while being nipped between those rollers. In such case, with regard to the pair of conveying rollers 110, the drive force of the pair of conveying rollers 110 is released by the use, for example, of a clutch or the like at a point of time whereat the leading edge of the recording material arrives at the transfer station 104, whereby the influence thereof can be eliminated. However, the pair of fixing rollers 101 are generally such that a pair of rollers are opposed and urged against each other and a recording material is conveyed while being nipped between the pair of rollers to thereby accomplish the fixation of an unfixed toner image and therefore, the conveyance force of the pair of fixing rollers 101 is usually very great as compared with the conveyance force of the photosensitive drum 102. Thus, where the same recording material is caught by both the transfer station 104 and the pair of fixing rollers 101, that is, where transfer is still effected in the trailing end portion of the recording material and the peripheral speed of the pair of fixing rollers 101 is higher than the peripheral speed of the photosensitive drum 102 when fixation is being effected on the recording material as the latter is nipped and conveyed between the pair of fixing rollers 101, the recording material is pulled by the pair of fixing rollers 101 and slips in the transfer station 104 and thus, image transfer misregistration occurs on the recording material.
FIG. 6 of the accompanying drawings shows a full color image forming apparatus using a plurality of image bearing members. In this apparatus, a recording material fed from a cassette 106 is electrostatically attracted to a conveyor belt 105 and passes photosensitive drums 120, 130 and 140 in succession, whereby cyan, magenta and yellow toner images are successively transferred from the respective drums onto the recording material to form an unfixed full color image thereon, whereafter the unfixed image is fixed by a pair of fixing rollers 101 and the recording material having a completed image output thereon is discharged out of the apparatus. Also, in this apparatus, if the peripheral speed of the pair of fixing rollers 101 becomes higher than the speed of the conveyor belt 105 during the image transfer at the transfer station 150 of the photosensitive drum 140 and the recording material is pulled by the pair of fixing rollers 101, the attraction of the recording material to the conveyor belt 105 will be weakened and the recording material will slip in this portion. Therefore, transfer misregistration will occur to the image transferred from the photosensitive drum 140 onto the recording material, and this has led to color misregistration on the images which means clear degradation of the quality of image.
In the above-described examples of the prior art, when the peripheral speed of the pair of fixing rollers 101 is higher than the feeding speed of the recording material at the transfer station 104 of the photosensitive drum 102 or the feeding speed of the recording material at the transfer station 150 of the photosensitive drum 140 and moreover the recording material lies between the pair of fixing rollers 101 and the transfer station 104 or 150, the recording material is pulled by the pair of fixing rollers 101 and slips at the transfer station 104 or 150, whereby transfer misregistration occurs on the recording material and causes the quality of image to be degraded.
As a means for solving this problem, there is the measure of making the distance between the transfer station 104 or 150 and the pair of fixing rollers 101 greater than the length of the recording material used with respect to the direction of conveyance. According to such measure, the distance between the transfer station 104 or 150 and the pair of fixing rollers 101 is made longer than necessary and undesirably, this leads to an increased dead space occupied by the entire apparatus. Also, the direction of conveyance of the recording material is often the widthwise direction of the apparatus and accordingly, making said distance long has led to a problem that the apparatus becomes bulky and the degree of freedom of the space in which the apparatus is installed is remarkably decreased.
Another means for solution is disclosed in Japanese Patent Application No. 186671/1985 filed by the same assignee. This means comprises stopping the pair of fixing rollers when the leading edge of the recording material enters the nip between the pair of fixing rollers, forming a loop in the recording material, thereafter rotating the pair of fixing rollers and thereby absorbing the difference between the speed of rotation of the pair of fixing rollers and the feeding speed of the recording material at the transfer station. However, according to this method, the pair of fixing rollers are stopped and this requires a clutch, for example, and the mechanical rising and falling times of this clutch affects the inter-paper spacing, and this provides a hindrance when a number of recording materials are conveyed continuously. Also, when a recording material enters the nip between the fixing rollers stopped, the resulting shock disturbs the unfixed toner image on the recording material and degrades the quality of the image and moreover, in the case of a full color image, said shock has resulted in the problem of color misregistration. Further, where the bus line of the photosensitive drum in the transfer station and the bus lines of the fixing rollers are not parallel to each other, there has also arisen a problem that the recording material is distorted to disturb the image thereon.